What Ignatius said.
However, the Church does have the right to make and repeal laws that bind us under the pain of sin, such as attending Mass on Sunday and Holy Days of Obligation, and the laws regarding marriage. These can change. For example, the Feast of the Annunciation is no longer a Holy Day of Obligation in the U.S.A. So while it would have been a sin to intentionally miss Mass on that feast day several decades ago, it no longer is.
But in the past the sin wasn’t really missing Mass on the Feast of the Assumption. Rather, it was disobeying the law of the Church regarding Holy Days of Obligation. A subtle but important distinction.
My point is that it may appear on the surface that things that were not sinful in the past now are, or vice-versa. However, you need to look deeper to understand whether it’s something that is intrinsically wrong, such as murder, or something that was disobedience to a changeable law, such as the Holy Days.
The sin is in disobedience to a legitimate and current law. Certain laws can change, and certain laws–those of a divine nature, such as murder–can never change.